Optical confirmation and redshift estimation of the Planck cluster candidates overlapping the Pan-STARRS Survey

Abstract

We report results of a study of Planck Sunyaev–Zel'dovich effect selected galaxy cluster candidates using the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) imaging data. We first examine 150 Planck-confirmed galaxy clusters with spectroscopic redshifts to test our algorithm for identifying optical counterparts and measuring their redshifts; our redshifts have a typical accuracy of σz/(1+z) ∼ 0.022 for this sample. Using 60 random sky locations, we estimate that our chance of contamination through a random superposition is ∼3 per cent. We then examine an additional 237 Planck galaxy cluster candidates that have no redshift in the source catalogue. Of these 237 unconfirmed cluster candidates we are able to confirm 60 galaxy clusters and measure their redshifts. A further 83 candidates are so heavily contaminated by stars due to their location near the Galactic plane that we do not attempt to identify counterparts. For the remaining 94 candidates, we find no optical counterpart but use the depth of the Pan-STARRS1 data to estimate a redshift lower limit zlim(1015) beyond which we would not have expected to detect enough galaxies for confirmation. Scaling from the already published Planck sample, we expect that ∼12 of these unconfirmed candidates may be real clusters.